OH Dipoles Research Articles

Theoretical characterization of the minimum energy path for the reaction H+O2→HO2*→HO+O

The potential energy surface for the reaction H+O2→HO*2 →HO+O has been characterized in the vicinity of the minimum energy path using CASSCF/contracted CI calculations with a basis set which is triple zeta valence quality plus three sets of polarization functions. CASSCF/CI calculations were carried out along the CCI minimum energy path. The latter calculation shows essentially no barrier for addition of an H atom to O2, in agreement with predictions made in earlier studies. The potential surface for recombination of OH and O is complicated by a crossing, at rOO ≈5.5a0, between the surface for electrostatic (OH dipole–O quadrupole) interaction and that for the formation of an O–O chemical bond. This surface crossing results in a small (≈0.5 kcal/mol) barrier.

High resolution infrared spectroscopy of hydrogen impurities in potassium tantalate

Abstract The high resolution infrared absorption related to the OH stretching mode (around 3500 cm−1) in KTaO3 is studied as a function of temperature. In two samples, we observed a splitting of each of the three high temperature peaks into up to four peaks at low temperature. The spectrum obtained is essentially the same for both samples, the small differences resulting from the different thickness of the two samples. The evolution of the absorption frequency as a function of temperature is followed for the three samples. The absorption frequency decreases with increasing temperature and a simple model, involving thermal expansion, is presented and successfully confronted to our datas. The multiplicity of the peaks is related to the presence of three inequivalent positions for the OH dipole giving rise to the three high temperature peaks. The low temperature splitting is related to the fluctuations of the ferroelectric soft modes as it was found in SrTiO3.

OH groups in nominally anhydrous framework structures: An infrared spectroscopic investigation of danburite and labradorite

The pleochroic behaviour of two nominally anhydrous structurally similar minerals, danburite and An59 labradorite, was investigated in the region of the OH stretching frequencies. Danburite shows a sharp absorption band at 3540 cm−1, labradorite shows a broad band with an absorption maximum at 3230 cm−1. On the basis of the pleochroic scheme of theinfrared (IR) absorption spectra it is proposed that the OH dipoles in danburite are located within the symmetry plane showing a distinct orientation parallel to [010]; the OH groups in labradorite are oriented approximately perpendicular to (001). The proposed models are in accordance with bond valence calculations showing that in both framework structures the most deficient oxygens, O5 in danburite and O C m in labradorite, are partially replaced by OH.

The pleochroism of a gem-quality enstatite in the region of the OH stretching frequency, with a stereochemical interpretation

The pleochroic behaviour of a gem-quality enstatite from Tanzania was investigated in the region\(\bar v\)=2500 to 4000 cm−1. Two sharp absorption bands at\(\bar v\)=3410 cm−1 and\(\bar v\)=3510 cm−1 are interpreted to be caused by OH stretching vibrations. As their absorption coefficients are considerably larger parallel to [001] (=direction of the silicate chains) than perpendicular to this direction, the OH dipoles have to be oriented approximately parallel to [001]. On this basis, a stereochemical interpretation of the incorporation of the OH groups into the structure is given.

NMR applied to minerals. V. The localization of vacancies in the octahedral sheet of aluminous biotites

The localization of vacancies in the octahedral sheet of aluminous biotites has been investigated by the use of 1H nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. The polytype to which the samples belonged was determined by x-ray diffraction methods. The joint use of all three techniques was essential in determining unambiguously the exact position of the vacancies and orientation of the associated O-H and Fe-H vectors. It was found that vacancies are located in the M1 (pseudo-centrosymmetric) sites and that contrary to what is usually assumed the OH dipoles are not, in this case, oriented directly towards the vacancy site.

Lithium in the interlayer space of synthetic trioctahedral micas

This paper gives several new and strong arguments in favour of the possibility of fixation of anhydrous lithium in the interlayer space of trioctahedral potassium micas. From the chemical viewpoint Li + can replace K +, but is located out of the alkaline cation site; it enters pseudo-octahedral cavities limited by the triangular bases of two aluminous tetrahedra of two consecutive sheets. The solubility limit of Li + in the interlayer is a function of the Al IV content of the mica. It is given by the relation ( Li Li + K) max = 2[( Al Si + Al) IV ] 2 . In both micas investigated — phlogopite, KMg 3(Si 3Al)O 10(OH) 2, and eastonite, K(Mg 2.5Al 1.5)(Si 2.5Al 1.5)O 10(OH) 2 — there is a remarkable agreement between the calculated values of the solubility limits and those measured by exchange reactions with hydrothermal solutions, at 600°C, 2 kbar. In high-Al micas, the interlayer Li content can be very important, with about one-third of K + replaced by Li +. The fixation of Li + according to this model provokes a strong flattening of the interlayer (strong decrease of the reticular distance d 005) and a slight increase of the reticular distance d 060. Infra-red (IR) absorption spectrometry shows that vacant K + sites are created when Li + enters the interlayer; one observes low-frequency OH stretching bands attributed to OH dipoles lying towards these empty sites. Fixation of Li + does not provoke any modification of the IR spectra in the region 1200-300 cm −1, indicating that Li + is really out of the sheet. For both cell dimensions and IR spectra, a comparison is made with “ordinary” lepidolites, having Li + in the octahedral sheet; it provides a guide for the distinction between the two species of Li-bearing micas.

Application of the semicontinuum potential model to deduce localized electron trapping sites in single crystals of d-sorbitol

The semicontinuum model of electron solvation [J. Am. Chem. Soc. 95, 1398 (1976)] has been used to calculate the stability of the trapped electron in single crystals of D-sorbitol at 4.2 K. Experimental ENDOR results suggest an environment of two OH dipoles, with the protons at equal distances from the electron. The geometry of the most probable site in a lattice undistorted by irradiation results in an unbound electron state. This arises because the dominant term in the total energy is the dipole–dipole interaction which is attractive for that geometry. If the oxygen atoms remain fixed at crystal lattice positions but the protons are allowed to rearrange so as to point towards the electron, a minimum in the total energy is obtained, but the electron–proton distance is much shorter than the experimental value. However, good agreement with experiment is obtained for configurations of two or four OH dipoles pointing towards the trapped electron when lattice rearrangement is allowed. These calculations suggest that dipole rearrangement occurs during electron trapping in single crystals even at 4.2 K.

The direction of OH dipoles in axinite: Comparison of the results from x-ray diffraction and infrared studies

Article The direction of OH dipoles in axinite: Comparison of the results from x-ray diffraction and infrared studies was published on November 1, 1975 in the journal Zeitschrift für Kristallographie - Crystalline Materials (volume 141, issue 1-6).

The direction of OH dipoles in axinite: Comparison of the results from x-ray diffraction and infrared studies

Article The direction of OH dipoles in axinite: Comparison of the results from x-ray diffraction and infrared studies was published on October 1, 1975 in the journal Zeitschrift für Kristallographie - Crystalline Materials (volume 141, issue 5-6).

Anisotropy of the Oxygen Kα X‐Ray Emission Band From Mg(OH)2, Single Crystals

AbstractThe 3σ and 1π molecular orbital contributions to the Oxygen Kα emission band of the hydroxyl ion can be separated by making use of the fact that, in the Mg(OH)2 structure, all OH dipoles are oriented parallel to the c‐axis. The O Kα component due with a 1π → 1s transition at 525.5 eV has its maximum intensity parallel to the crystallographic c‐direction [001], while the 3σ→ 1s component is emitted mainly in the zone (hkO), normal to the c‐direction. The 3σ→ 1s component appears to be split due to electron‐proton interaction, exhibiting two maxima at 524.7 and 526.0 eV.

Photoelectron emission from Mg(OH)2 single crystals in the valence band region

Abstract The x-ray stimulated photoelectron emission from a) the (10 1 0) and b) the (0001) surface of Mg(OH)2 single crystals was studied. A marked anisotropy was observed in the valence band region. For a) three maxima were found at 4.6, 8.8 and 12.0 eV which can be assigned to the T1u, Eg and A1g states respectively, if regular Oh symmetry is assumed for the ligand field. For b) the A1g band disappears which is attribu to the fact that in Mg(OH)2 the octahedral symmetry is reduced to D3d by the special orientation of the OH dipoles parallel to the c-axis.

Infrared Emission of OH in the Fundamental and First Overtone Bands

The infrared vibration–rotation spectrum of the hydroxyl radical produced by the reaction of hydrogen atoms and ozone has been recorded. The intensity of individual lines in the fundamental sequence was found to be considerably weaker than was previously predicted. The intensity of lines in the fundamental sequence is compared with the intensity of the first overtone lines to obtain ratios of the vibrational transition probabilities. These ratios are used to remove the ambiguity of the OH dipole moment function associated with previously unknown intensities of the fundamental bands. A complete set of relative transition probabilities is given. An equilibrium flux calculation using atmospheric data is performed using the derived transition probabilities to obtain the vibrational distribution of OH at formation.

Perturbation of OH groups in decationated Y-zeolites by physically adsorbed gases. Part 2.—Heats of adsorption

The shift and the change in intensity of the OH high frequency stretching band of a decationated near faujasite Y-zeolite have been correlated to the heats of physical adsorption of Ar, Kr, O2 and CH4. The initial isosteric heats of adsorption of these gases decrease with increasing polarizabilities of the adsorbates. This unusual behaviour is tentatively explained by assuming that the potential energy of adsorption involves an additional positive term corresponding to the increase of the OH dipole moment. The reaction field arising from the change of the dielectric constant in the zeolitic cavity should be responsible for this increase. This simple electrostatic model allows one to correlate the infra-red to the thermodynamic data.

Einstein A Coefficient for the Λ Doublet Transitions of the Ground State of OH

FOR some time now there has been some uncertainty in the literature of the correct value for the Einstein A coefficients for the ground state Λ doublet transitions of OH. Barrett1 calculated A = 2.66 × 10−11 sec−1 for the 1,667 Mc/s line using a matrix element |μij|2 taken from Dousmanis, Sanders and Townes2. This element is incorrect both because the form of its dependence on the rotational quantum number J was incorrectly given as [(J + 1)(2J + 1)]−1, and also because the element was incorrectly defined as (1). The appropriate definition is given below. The correct J-dependence, which is [J(J + 1)]−1, was given by Meyer3, in connexion with the Stark-effect determination of the OH dipole moment, μ because Meyer used an electric field directed so that only transitions of type ΔMJ = 0 occurred between the magnetic sub-levels characterized by quantum number MJ, the problem of combining these elements with those for which ΔMJ = ±1 to form an overall matrix element μij was not encountered. To derive the overall matrix element between the Λ doublet levels of the ground state of OH, Goss and Weaver4 combined the correct J-factor of Meyer with standard relative intensity formulae for hyperfine transitions as given by Townes and Schawlow5 to derive A = 0.9640 × 10−11 sec−1 for the 1,667 Mc/s line. They also used the most recent values for the OH dipole moment and the fine structure interaction constant, the calculation making use of the theory for intermediate coupling between Hunds's cases (a) and (b), as given by Dousmanis, Sanders and Townes2.

Hyperfine Structure inO17H and the OH Dipole Moment

A microwave spectrometer for the study of free radicals or other reactive species is described; it can be used with either Stark or Zeeman modulation. This spectrometer was used to determine the hyperfine structure of ${\mathrm{O}}^{17}$H. Four main lines of the $^{2}\ensuremath{\pi}_{\frac{3}{2}}$, $J=\frac{7}{2}$ $\ensuremath{\Lambda}$-doubling transition were definitely observed, and there is evidence that one satellite line was also detected. Quantum number assignments were made for the four main lines, which are divided into two doublets. In the course of establishing the uniqueness of these assignments, the $\ensuremath{\Lambda}$-doubling frequency for the $^{2}\ensuremath{\pi}_{\frac{3}{2}}$, $J=\frac{7}{2}$ state of ${\mathrm{O}}^{18}$H was experimentally determined. The separation between doublets in the ${\mathrm{O}}^{17}$H spectrum was used to determine the hyperfine constant $d$ associated with the ${\mathrm{O}}^{17}$ nucleus. The constant $d$ was also computed theoretically from a simple molecular model of OH in which a single unpaired $p\ensuremath{\pi}$ electron about the ${\mathrm{O}}^{17}$ nucleus is assumed. The experimental value is about 8% greater in magnitude than the theoretical value, and this difference is discussed. The same spectrometer was used for the determination of the electric dipole moment of OH by measuring the Stark shift of the $^{2}\ensuremath{\pi}_{\frac{3}{2}}$, $J=\frac{7}{2}$, $M=4$ $\ensuremath{\Lambda}$-doubling line of ${\mathrm{O}}^{16}$H at a known electric field. The OH dipole moment was found to be 1.60\ifmmode\pm\else\textpm\fi{}0.12 D.